The exhaust noise produced by two cycle engines presents a serious and difficult problem. The exhaust noise problem has become even more serious in recent years with the development of high compression, high performance and high speed engines. By reason of increased cylinder pressure the "blowdown" pulse that occurs upon opening of the exhaust port of the engine has become sharper and of greater amplitude thereby increasing the noise level of the exhaust. When used in populated areas the noise of small two cycle engines has become particularly objectionable. Moreover, it is of such a high level that permanent ear damage may be caused. Legislation has been enacted in some countries and is being considered in others to place a legal limit on the noise level of internal combustion engines.
Heretofore efforts have been made to decrease the exhaust noise of internal combustion engines by designing more effective mufflers attached to the exhaust passages of the engines. However, this approach has its limitations. In general, a more effective muffler decreases the power output of the engine by impeding the flow of exhaust gases and thereby building up a back pressure. Moreover, with hand held power implements such as chain saws, the attenuation of exhaust noise presents further problems by reason of space, weight and cost factors. In small engines such as those used on such hand held implements, space and weight are at a premium by reason of a continued effort to make the engines as small and light as possible. In order effectively to attenuate the exhaust noise of such an engine, a muffler might be as large as the rest of the engine, and weigh almost as much. Moreover, as chain saws and other power operated implements are highly competitive the additional cost involved in manufacturing a highly effective muffler is undesirable from the point of view of production costs.